Various functional structures and coatings are combined with ophthalmic substrates to impart or modify ophthalmic lens characteristics, other than optical power or magnification characteristics, Some examples of functional properties include light polarization, photochromism, tint, color, decor, hardness, and abrasion resistance.
The functional structures can be adhesively bonded to the lens by lamination onto the ophthalmic substrate. Alternatively the ophthalmic substrate can be casted or injection molded onto the functional structure.
Such functionalized ophthalmic lenses can undergo additional processing such as grinding to prescription strength and/or be subjected to additional coating processes for mechanical or optical enhancement, as described in EP1175280. Surface machining and surface smoothing create high mechanical pressure and stress on lenses, which may release molded-in residual stresses. Coating can be done in many ways. but usually a lens undergoes several steps: cleaning in chemical baths, coating with radiation and/or heat curable compositions and curing. Heating lenses may also release molded-in residual stresses. Other processing steps may be used, like annealing, lamination, tinting, edging, mounting.
Such additional processing steps may result in warpage of the lens. Warpage is a distortion where the surfaces of the molded part do not follow the intended shape of the molding design. Warpage results from molded-in residual stresses and anisotropic materials properties which are released during further processing steps like surfacing or coating. Warpage is identified by the bending of the lens in one direction more than the other, which deforms the lens like a “potato chip”, resulting in a change in lens curvature.
Warpage is a well-known phenomenon in molding industry. In optics, molded objects like memory disks or contact lenses are very sensitive to shape distortions. Molding processes have thus been developed to avoid warpage, in which applied pressure, cooling rate and duration of each steps are precisely defined. By this way, molded-in residual stresses are limited by a proper process design. However, such process conditions are slow and limit productivity of molding machines. In addition, these processes do not apply well to composite molded products, in which two different materials with different dilatation rates are in direct contact.
Ophthalmic lenses comprising a thermoplastic substrate and a light polarizing structure are well known. To ensure a good adhesion between these elements various solutions have been developed. For instance, adhesives are laid between substrate and light polarizing structure. Such adhesives are often sensitive to water or oily materials and have poor optical properties. Alternatively, the external layer of the light polarizing structure is selected so as to fuse with the thermoplastic substrate. When the same material is used as thermoplastic substrate and external layer, there is no more interface: delamination is avoided but warpage can still occur because polarizing structure (very often a highly stretched film) presents residual stresses.
After intensive research the inventors have developed an ophthalmic lens that does not show the drawbacks mentioned above.